Process for evaluating the remaining charge in an accumulator battery

ABSTRACT

A process is disclosed for evaluating the remaining charge in an accumulator battery by determining the residual charge at the start of discharge, adding up the quantities of energy extracted as and when the battery is used, determining the quantities of charge which are not restorable instantaneously because of a high discharge rate, charge which is restorable by relaxation of the battery and charge which is not restorable under the influence of temperature.

The present invention relates to a process for evaluating the remainingcharge in an accumulator battery.

A self-contained electric traction vehicle cannot be used in an urbanarea unless the remaining endurance which its traction accumulatorbatteries can confer upon it is known as accurately as possible. Theremaining endurance of the vehicle corresponds to the distance or timefor which the car can run before exhausting the electric chargecontained in these traction accumulator batteries. A knowledge of theremaining self-containment of the vehicle relies on a knowledge of twofactors. The first of these factors, which is subjective, concerns theconsumption of the vehicle over the trips ahead. This consumptiondepends in particular on the profile of the journey, on the density oftraffic and on the driver's way of driving. The second factor, which isobjective, concerns the charge restorable at each instant by thetraction battery.

The existing devices which indicate the restorable charge, and which arecalled gauges, are based on measurement of the voltage across theterminals of the battery (Curtis type devices, etc.). The accuracy ofthese devices is less than 20% of the nominal capacity for a newbattery, and deteriorates considerably on aging: the error of estimationcan exceed 50%.

The subject of the present invention is a process for evaluating theremaining charge in an accumulator battery, which makes it possible toevaluate the remaining charge with the best possible accuracy,regardless of the conditions of use of the battery and regardless of itsage.

The process according to the invention, for evaluating the remainingcharge in an accumulator battery, according to which the residual chargeat the start of discharge is determined and the quantities ofelectricity extracted as and when the battery is used are added up, ischaracterized in that account is taken of the quantities of charge whichare not restorable instantaneously under the influence of the dischargerate and of the charge which is restorable by reducing the intensity ofthe current drawn from the battery.

The present invention will be better understood on reading the detaileddescription of an embodiment taken as non-limiting example andillustrated by the appended drawing in which:

FIG. 1 is a graph of the restorable charge of a battery, as a functionof temperature, and

FIG. 2 is a graph of the restoring of the charge which is not restorableinstantaneously of a battery as a function of time;

FIG. 3 is a graph of the changes in a coefficient involved in theformula giving the portion of the charge of a battery which is notrestorable under the influence of the discharge rate, as a function ofdischarge current;

FIGS. 4 to 8 are examples of curves used by the process of theinvention.

The invention is described below with reference to a tractionaccumulator battery for electric vehicle, but obviously it is notlimited to such a type of accumulator and may be implemented for varioustypes of accumulator, from the simple accumulator for small portableapparatus up to a large back-up accumulator set for stationary plant ofhigh power. These accumulators can be of various technologies: lead,cadmium-nickel, etc. For simplicity, in the remainder of the text, theterm battery will be used to designate any one of these accumulators orsets of accumulators.

The operating modes envisaged for the battery are as follows: discharge,charging when stationary, charge recovery by braking (valid only for theapplication to vehicles with energy recovery from braking ordeceleration), and self-discharge during storage without use. To each ofthese modes of operation of the battery there corresponds adetermination of the charge restorable by the battery, described below.

Mode of operation in the discharge regime:

Calculation of the charge remaining in a battery during the dischargephase amounts to calculating, for a set of consecutive time spans Δt,the value Ch.sub.(t) whose mathematical expression (1)is given a littlelater on.

In the expression (1), Ch.sub.(ti) denotes the initial charge containedin the battery at the start of the discharge phase, I.sub.(t) denotesthe current provided by the battery and measured at each instant Δt,Ch.sub.(t) denotes the charge contained in the battery at each instant,and T.sub.(t) denotes the temperature of the battery, measured at eachinstant.

Calculation of Ch.sub.(t) depends on five terms which are related to thephysico-chemical behaviour of the battery. These terms are:

1) the residual charge at the start of discharge;

2) the quantity of electricity consumed, measured in series with thebattery (measurement of the current provided by the battery throughoutthe entire time of use);

3) the portion of the charge of the battery which is not restorableunder the influence of temperature;

4) the portion of the charge which is not restorable under the influenceof the battery discharge rate;

5) the portion of the charge restored through the relaxation of thebattery.

Of these five terms, the first two are used by the known processes,whereas the other three are specific to the process of the invention.

First term: residual charge at the start of discharge=Ch.sub.(t-Δt) :this is the value of the charge available in the battery and calculatedat the calculation step Δt preceding the relevant instant t. At thestart of the discharge phase it is equal to Ch.sub.(ti).

Second term: quantity of electricity consumed measured for a battery:

    -I.sub.(t) *Δt

this is the quantity of electricity extracted from the battery duringthe calculation step Δt.

Third term: portion of the charge not restorable under the influence oftemperature:

    f1(Ch.sub.(t), T.sub.(t)):

this is the function characterizing the charge which the battery cannotrestore owing to the ambient temperature. Mathematically it representsthe fact that at low temperature the phenomena of diffusion of chargeswithin the electrolyte are slowed down and render charges inaccessibleduring discharge.

It is expressed in the general form:

    F1(Ch.sub.(t) T.sub.(t))=K.sub.1 (T.sub.(t))/100*Ch.sub.(t)

for example, for a sealed lead traction battery: a formula is used ofthe form (approximation by linear segments--see FIG. 1):

    for T.sub.(t) ≦30° C.: K.sub.1 (T.sub.(t))=μ1*T.sub.(t) +μ2

    for 10° C.≦T.sub.(t) ≦30° C.: K.sub.1 (T.sub.(t))=μ3*T.sub.(t) +μ4

    for +10° C. ≧T.sub.(t) : K.sub.1 (T.sub.(t) =μ5*T.sub.(t) +μ6

Fourth term: portion of the charge not restorable under the influence ofthe discharge rate:

    -f2[Ch.sub.(t), T.sub.(t), I.sub.(t) ]*Δt

This is a function characterizing the instantaneous efficiency (K2) ofdischarge, that is to say the instantaneous charge (during the incrementof time Δt) which the battery cannot restore owing to the currentdemanded by the consumer connected up to the battery (for example: atraction engine). Mathematically it represents the battery "stress" thatis to say the fact that at high current the phenomena of diffusion ofcharges within the electrolyte are not fast enough to bring close to theelectrodes charges capable of continuing to supply the consumer circuit.These charges which are not immediately available are present in thebattery and will become available during periods of relaxation of thebattery.

It is expressed in the general form:

    f2(Ch.sub.(t), T.sub.(t), I.sub.(t))=[I.sub.(t) *100/{100 -K.sub.2 (T.sub.(t), I.sub.(t), Ch.sub.(t))}]-I.sub.(t)            (A)

for example, for a sealed lead traction battery, use is made of apiecewise linearization of a formula of the form:

    for I.sub.(t) ≦32A we have K.sub.2 (I.sub.(t), T.sub.(t), Ch.sub.(t))=0

    for -10° C.≦T.sub.(t) ≦50° C. et 32A≦I.sub.(t) ≦300 A:

    K.sub.2 (T.sub.(t), I.sub.(t))=+μ8-e.sup.-μ7(T.sbsp.(t))*I.sbsp.(t)(B)

(see FIG. 3).

Fifth term: portion of the charge restored by relaxation of the battery:(no usage of the battery or sharp reduction in the discharge current)

    +f3[-f2(Ch.sub.(t-Δt), T.sub.(t-Δt), I.sub.(t-Δt))+Chre cov.sub.(t), T.sub.(t), I.sub.(t) ]*Δt and

    Chre cov.sub.(t) =f3[-f2(Ch.sub.(t-2Δt), T.sub.t-2Δt), I.sub.(t-2Δt) +Chre cov.sub.(t-Δt), T.sub.(t-Δt), I.sub.(t-Δt) ]

This is a function characterizing the relaxation of the battery duringdischarge, that is to say the instantaneous charge recovered during theincrement of time Δt and which could not be restored during thepreceding calculation steps (Δt) owing to the stress of the battery.Mathematically it represents the battery "relaxation", that is to saythe fact that the phenomena of diffusion of charges within theelectrolyte make it possible progressively to bring charges which werenot previously restored close to the electrodes so as to supply theconsumer circuit. This charge converges to 0 with time.

It is expressed in the general form:

    F3[-f2(Ch.sub.(t-Δt), T.sub.t-Δt), I.sub.(t-Δt))+Chre cov.sub.(t), T.sub.(t), I.sub.(t) ]=1-e.sup.-[K3{T.sbsp.(t), I.sbsp.(t)}]*[-f2(Ch.sbsp.(t-Δt), T.sbsp.(t-Δt, I.sbsp.(t-Δt))+Chre cov.sbsp.(t)]                   (C)

with:

    Chre cov.sub.(t) =f3[-f2{Ch.sub.(t-2Δt), T.sub.(t-2Δt), I.sub.t-2Δt) }+Chre cov.sub.(t-Δt), T.sub.(t-Δt), I.sub.(t-Δt) ]

for example, for a sealed lead traction battery use is made of apiecewise linearization of a formula of the form:

    for -10°≦T.sub.(t) ≦50° C. et 0A≦I.sub.(t) ≦300 A:

    f3[-f2(Ch.sub.(t-Δt), T.sub.(t-Δt), I.sub.(t-Δt))+Chre cov.sub.(t), T.sub.(t), I.sub.(t) ]

in this expression, f2 has the value given by the relation (A), K2 hasthe value given by the relation (B) and f3 has the value given by therelation (C).

Mathematical form for evaluating the charge available in the battery atthe instant t: Ch.sub.(t).

Let Ch.sub.(ti) be the initial charge contained in the battery at thestart of the discharge phase.

Let I.sub.(t) be the current delivered by the battery and measured ateach instant.

Let Ch.sub.(t) be the charge contained in the battery at each instant.

Let T.sub.(t) be the battery temperature measured at each instant.

Calculation of Ch.sub.(t) is carried out according to the formula:##EQU1##

This formula is expressed in discrete form as follows:

let Δt be the period of calculation of the evaluation of Ch.sub.(t) :

at t=t0: Ch.sub.(t) =Ch.sub.(ti) -f1(Ch.sub.(ti), T.sub.(t0))

at t=t0+Δt we have:

    Ch.sub.(t) =Ch.sub.(t0) -f1(Ch.sub.(t0), T.sub.(t)))-I.sub.(t) *Δt-f2(Ch.sub.(t), T.sub.(t), I.sub.(t) *Δt

at t=t0+2Δt we have:

    Ch.sub.(t) =Ch.sub.(t-Δt) -f1(Ch.sub.(t-Δt), T.sub.(t)))-I.sub.(t) *Δt-f2(Ch.sub.(t), T.sub.(t), I.sub.(t))*Δt +f3[-f2{Ch.sub.(t-Δt), T.sub.(t-Δt), I.sub.(t-Δt) }, +Chre cov.sub.(t), T.sub.(t), I.sub.(t))]*Δt

Let us call

    Chre cov.sub.(t) =f3[-f2{Ch.sub.(t-Δt), T.sub.t-Δt), I.sub.t-Δt)}+Chre cov.sub.(t-Δt), T.sub.(t), I.sub.(t))]

at t=t0+3Δt we have:

    Ch.sub.(t) =Ch.sub.(t-Δt) -f1[Ch.sub.t-Δt, T.sub.(t) ]-I.sub.(t) *Δt-f2(Ch.sub.(t), T.sub.(t), I.sub.(t)))*Δt+f3[-f2{Ch.sub.t-Δt), T.sub.(t-Δt), I.sub.(t-Δt) }-Chre cov.sub.(t), T.sub.(t), I.sub.(t) ]*Δt

at t=t0+nΔt we have:

    Ch.sub.t0+nΔt) =Ch.sub.(t-Δt) -f1(Ch.sub.t-Δt), T.sub.(t)))-I.sub.(t) *Δt-f2*Ch.sub.(t), T.sub.(t), I.sub.(t))*Δt

    +f3[-f2{Ch.sub.t-Δt), T.sub.(t-Δt), I.sub.(t-Δt) }+Chre cov.sub.(t), T.sub.(t), I.sub.(t) ]*Δt

Method for constructing the elements of the previous formula:

A) Expression: Ch.sub.(t-Δt):

this value is either obtained at the previous calculation step duringthe discharge mode, or arises out of the calculations for evaluating thecharge during the other modes of operation of the battery.

B) Expression: f(Ch.sub.(t), T.sub.(t)):

f(Ch.sub.(t), T.sub.(t)) is obtained by experimentally plotting thefollowing sets of curves:

1^(st)) V.sub.(t) =g (Ch.sub.(t), T.sub.(t)). To establish these curves,the parameters are measured with a constant temperature throughoutdischarge and a rate which is constant over time and is less than orequal to the nominal discharge rate for the type of battery considered(for example I₅) with: V.sub.(t) =voltage across the terminals of thebattery, T.sub.(t) =battery temperature measured for example on one ofthe output electrodes and I₅ =discharge current for complete dischargein five hours. Measurement is repeated for various temperatures. Anexample of such a set of curves is depicted in FIG. 4 for variousconstant discharge currents ranging from 300 A to 25 A, the numericalvalues being mentioned purely as indication, as is the case for all thefollowing figures.

2^(nd)) V_(stop) =h (I) gives the change in the minimum voltage acrossthe terminals of the battery as a function of the discharge currents.(See FIG. 5).

From these two curves is calculated the curve

    (Ch.sub.(t) -C.sub.max(t))/C.sub.max(t) =K.sub.1 (T.sub.(t)) pour V.sub.(t) =V.sub.arret =h(Ie)

where Ie is the discharge rate used to obtain the curves V.sub.(t)=g[(Ch.sub.(t), T.sub.(t) ]. C_(max)(t) is measured for a discharge atnominal temperature (for example 25° C.) and with a rate which is lessthan or equal to the nominal discharge rate for the type of batteryconsidered (for example I₅). This discharge is preceded directly byoptimal recharging allowing full charging of the battery. (For examplefor a lead battery a charge of the type IUi will be applied underoptimal conditions (temperature=25° C.)). The charges Iui are, for leadbatteries, charges during which the current injected into the batteryand the voltage across the terminals of the battery are monitored overtime for the purpose of maximizing the charge stored in a batterywithout destroying or impairing it.

C) Expression: I.sub.(t) *Δt:

I.sub.(t) is measured in series with the battery with a samplingfrequency which is compatible with the dynamics of consumption (forexample on a vehicle, with the dynamics of driving).

D) Expression: f2(Ch.sub.(t), T.sub.(t), I.sub.(t))*Δt: f2(Ch.sub.(t),T.sub.(t), I.sub.(t)) is obtained by experimentally plotting thefollowing sets of curves:

1^(st)) V.sub.(t) =g [Ch.sub.(t), I.sub.(t) ]. The parameters aremeasured at I.sub.(t) constant over time and at nominal temperature (forexample 25° C.) then at various temperatures which are constant overtime and distributed over the range of operation of the battery (forexample for a sealed lead traction battery -20° C. to +60° C.). (Seealso FIG. 4). V.sub.(t) is the voltage across the terminals of thebattery, T.sub.(t) is the battery temperature measured for example onone of the output electrodes.

2^(nd)) V_(stop) =h(I) gives the change in the minimum voltage acrossthe terminals of the battery as a function of the discharge currents.

a) From the curves g [Ch.sub.(t), I.sub.(t) ] measured at nominaltemperature and h.sub.(I), is calculated the curve:

    [Ch.sub.(t) -C.sub.max(t) ]/C.sub.max(t) =K2(I.sub.(t)) pour V.sub.(t)=V.sub.arret =h (Ie)

where Ie is the discharge rate used to obtain each curve of the setV.sub.(t) =g (Ch.sub.(t), I.sub.(t)).

C_(max)(t) is measured for a discharge at nominal temperature (forexample 25° C.) and with a rate which is less than or equal to thenominal discharge rate for the type of battery considered (for exampleI₅). This discharge is preceded directly by optimal recharging allowingfull charging of the battery. (For example for a lead battery a chargeof the type IUi will be applied under optimal conditions(temperature=25° C.)).

b) From the curves g (Ch.sub.(t), I.sub.(t)) measured at varioustemperatures and h(I), are calculated the various curves [Ch.sub.(t)-C_(max)(t) ]-C_(max)(t) =K2 (I.sub.(t)) for V.sub.(t) =V_(stop) =h(Ie)where Ie is the discharge rate used to obtain each curve of the setV.sub.(t) =g (Ch.sub.(t), I.sub.(t).

The change in the coefficients of K2 (I.sub.(t)) is calculated as afunction of the temperature variation. For example for a sealed leadtraction battery we obtain:

    K2(I.sub.(t), T.sub.(t))=+μ8[-e.sup.[-μ7(T.sbsp.(t))*I.sbsp.(t)]

    with μ7(T.sub.(t))=-Ω.sub.1 *T.sub.(t) +Ω.sub.2 for T.sub.(t) ≧25° C.

    with μ7)(T.sub.(t))=-Ω.sub.3 *T.sub.(t) +Ω.sub.4 for T.sub.(t) ≦25° C.

E) Expression:

    +f3 [-f2 (Ch.sub.(t-Δt), T.sub.(t-Δt), I.sub.(t-Δt) ]+Chre cov.sub.(t), T.sub.(t), I.sub.(t))*Δt

and expression:

    Chre cov.sub.(t) =f3[-f2{Ch.sub.(t-Δt), T.sub.(t-Δt), I.sub.(t-Δt) }+Chre cov.sub.(t-Δt), T.sub.(t), I.sub.(t) ]:

    The part f3[-f2{Ch.sub.(t-Δt), T.sub.(t-Δt), I.sub.(t-Δt) }+Chre cov.sub.(t), T.sub.(t), I.sub.(t))]

is obtained by experimentally plotting the following sets of curves (seeFIG. 6):

1^(st)) *V.sub.(t) =m(Ch.sub.(t), T.sub.(t)) with modification ofI.sub.(t) over time:

from t=t0 to t=t1, discharge with current I₁

from t=t1 to t=t2, discharge with current I₂

from t=t2 to t3=end of discharge, discharge with current I constant overtime and less than or equal to the nominal discharge current for thetype of battery considered (for example I₅).

These curves are firstly plotted at nominal temperature while firstlyvarying the parameters t1 and t2 between t0 and the end of discharge,while fixing I₁ at the nominal discharge rate and I₂ at 0A.

New curves are plotted at nominal temperature by firstly varying theparameters t1 and t2 between t0 and the end of discharge, while fixingI₁ at various discharge rates and I₂ at 0 A.

New curves are plotted at nominal temperature by fixing t1 and t2between t0 and the end of discharge (for example t1≈3/5 of the expecteddischarge time and t2≈4/5 of the expected discharge time), and by fixingI₁ at various discharge rates and I₂ at various discharge rates.

New curves are plotted at nominal temperature by fixing t1 and t2between t0 and the end of discharge (for example t1≈3/5 of the expecteddischarge time and t2≈4/5 of the expected discharge time), and by fixingI₁ at various discharge rates and I₂ at 0.

V.sub.(t) is the voltage across the terminals of the battery, T.sub.(t)is the battery temperature measured for example on one of the outputelectrodes.

2nd) *V_(stop) =h(I) gives the change in the minimum voltage across theterminals of the battery as a function of the discharge currents.

a) From the curves m [Ch.sub.(t), I.sub.(t) ] measured at nominaltemperature and h(I), is calculated the curve Chre cov.sub.(t) =K3 (I₁,I₂, Ch.sub.(t)) by considering:

t=end of discharge time fixed at the instant at which V.sub.(t)=V_(stationary) =h (Ie) with Ie=discharge rate of the third sequence ofthe experiment conducted for each curve of the set V.sub.(t)=m[Ch.sub.(t), I.sub.(t) ].

Ch.sub.(t) is measured by accounting the quantities of electricityflowing across the terminals of the battery.

This discharge is preceded directly by optimal recharging allowing fullcharging of the battery. (For example for a lead battery a charge of thetype IUi will be applied under optimal conditions (temperature=25° C.)).

b) From the curves m [Ch.sub.(t), I.sub.(t) ] measured at varioustemperatures and h(I), is obtained Chre cov.sub.(t) =K3 (I₁, I₂,Ch.sub.(t)) by considering t=end of discharge time fixed at the instantat which V.sub.(t) =V_(stationary) =h(Ie) with Ie=discharge rate of thethird sequence of the experiment conducted for each curve of the setCh.sub.(t) =m [Ch.sub.(t), I.sub.(t) ].

The change in the coefficients of K3 (I₁, I₂, Ch.sub.(t) ] is calculatedas a function of the temperature variation (note: as a firstapproximation the formula calculated above can be used for theexpression D: f2 (Ch.sub.(t), T.sub.(t), i.sub.(t)).

Mode of operation: charging when vehicle is stationary

Calculation of the charge stored in a battery during the phase ofcharging when stationary (that is to say with the help of a charger) isaccomplished in two possible ways:

1) If charging is performed by an intelligent optimal charger capable ofemitting an "end of charging and battery full" signal, then Ch.sub.(t)is fixed at the value of the maximum capacity of the battery at theinstant t: C_(max)(t). For example for a sealed lead traction battery, amicroprocessor-controlled intelligent charger IUi is used, emitting thetwo signals, charger present and battery fully recharged.

2) In all other cases, the battery energy management processor adds upat each Δt the quantity of current which arrives at the terminals of thebattery and multiplies it by an efficiency factor depending on thequantity of electricity contained in the battery at the instant t, onthe temperature and on the rate (intensity of charging). This gives aformula of the form:

    Ch.sub.(t) =Ch.sub.(t-Δt) +I.sub.(t) *Δt*f4(Ch.sub.(t), T.sub.(t), I.sub.(t))

The expression: f4(Ch.sub.(t), T.sub.(t), I.sub.(t)) is determinedexperimentally by taking the following readings (see FIG. 7):

a) At nominal temperature, standard charging is performed up toX1%*C_(max)(t) then discharging under nominal conditions of temperatureand of discharge current and the quantity of electricity gathered ismeasured. The experiment is repeated for various values of X1 and thefunction: charging efficiency=charge restored/charge stored is plotted.

b) The experiments are repeated for various temperatures, then forvarious charging intensities or profiles and the coefficients aredefined which correct the function calculated or approximated during theexperiments at nominal temperature.

Mode of operation: charge recovery by braking:

Calculation of the charge stored up in a battery during the chargerecovery by braking phase, that is to say for example with the aid of anengine converted into a generator during vehicle braking) [sic] isaccomplished as follows:

The battery energy management processor adds up at each increment Δt thequantity of current which arrives at the terminals of the battery andmultiplies it by an efficiency factor depending on the quantity ofelectricity contained in the battery at the instant t, on thetemperature and on the rate (charge recovery intensity). This gives aformula of the form:

    Ch.sub.(t) =Ch.sub.(t-Δt) +I.sub.(t) *Δt*f5(Ch.sub.(t), T.sub.(t), I.sub.(t))

The expression f5(Ch.sub.(t), T.sub.(t), I.sub.(t)) is determinedexperimentally by taking the following readings (see FIG. 8):

a) At nominal temperature: on a battery charged to X2%*C_(max)(t),several partial chargings and then dischargings of value: X0%*C_(max)(t)are performed an odd number of times (charging n times followed bydischarging then partial charging n+1) then discharging is carried outunder nominal conditions of temperature and of discharge current and thequantity of electricity gathered is measured. The experiment is repeatedfor various values of X2 and the function: charging efficiency=chargerestored/charge stored is plotted.

b) The experiments are repeated for various temperatures, then forvarious values of X3%, then various charging currents and thecoefficients are defined which correct the function calculated orapproximated during the experiments at nominal temperature.

Mode of operation: self-discharge during storage without use

Calculation of the charge lost from a battery during the recovery phaseof storage without use is accomplished as follows:

At each end of storage (hence at the start of each of the precedingmodes of operation when it is preceded by a phase in which the currentdelivered by the battery is zero) the battery energy managementprocessor evaluates the quantity of current lost during storage as afunction of the quantity of electricity contained in the battery at theinstant t, the temperature and the duration of storage. This gives aformula of the form:

    Ch.sub.(t) =Ch.sub.(t-NΔt) -f6(Ch.sub.t-NΔt) T, NΔt)

The expression: f6(Ch.sub.(t-NΔt), T, NΔt) is determined experimentallyby taking the following readings:

a) At nominal temperature: on a battery charged to X4%*C_(max)(t),storage for N1*Δt then discharge under the nominal conditions oftemperature and of discharge current and measurement of the quantity ofelectricity gathered. The experiment is repeated for various values ofN1 and the charge loss function is plotted.

b) The experiments are repeated for various temperatures, then forvarious values of X4% and the coefficients are defined which correct thefunction calculated or approximated during the experiments at nominaltemperature.

We claim:
 1. Process for evaluating the remaining charge in anaccumulator battery, according to which the residual charge at the startof discharge is determined and the quantities of electricity extractedas and when the battery is used are added up, wherein account is takenof the quantities of charge which is not restorable instantaneouslyunder the influence of the discharge rate and of the charge which isrestorable by reducing the intensity of the current drawn from thebattery.
 2. Process according to claim 1, wherein when evaluating theremaining charge, account is also taken of the portion of charge whichis not restorable under the influence of temperature.
 3. Processaccording to claim 1, wherein the portion of charge which is notrestorable under the influence of the discharge rate is a function, f2,of the form:

    f2(Ch.sub.(t), T.sub.(T), I.sub.(t)))=[I.sub.(t) *100/{100-K2(T.sub.(t), I.sub.(t), Ch.sub.(t) }]-I.sub.(t)

Ch.sub.(t) being the charge contained in the battery at each instant,T.sub.(t) being the battery temperature measured at each instant,I.sub.(t) being the intensity of the current delivered by the batteryand measured at each instant, and K2 being a parameter representing theinstantaneous efficiency of discharge.
 4. Process according to claim 3,wherein the portion of the charge restored by relaxation of the batteryis of the form:

    Chre cov.sub.(t) =f3[-f2(Ch.sub.(t-2Δt), T.sub.(t-2Δt), I.sub.(t-2Δt) +Chre cov.sub.(t-Δt), T.sub.(t-Δt), I.sub.(t-Δt)]

f3 being an exponential function of the recoverable energy in thebattery at a given instant and Δt a calculation step.